1. Field of the Invention
The present invention relates to a variable gain wideband amplifier to be used as a low noise amplifier in a wideband system. More particularly, the invention relates to a feedback or feedforward type variable gain wideband amplifier.
2. Description of the Related Art
Currently, communication systems are required to operate at least two different frequency channels owing to increasing multiband implementation. When applied to a communication system using different frequency channels, an amplifier has to amplify different frequency channels in wideband. In addition, since all existing amplifiers have nonlinear characteristics, it is required to provide means for reducing third order Inter-Modulation Distortion (IMD3) originating from the interference of different frequency channels.
One of conventional feedback variable gain amplifiers is shown in FIGS. 1 and 2.
FIG. 1 is a circuit diagram illustrating a conventional variable gain amplifier, which includes an amplifier circuit AMP1 for amplifying an input signal and a gain controller GC1 feedback-connected to the amplifier circuit AMP1 to conduct gain control.
The amplifier circuit AMP1 includes an inductor L11, which is connected between power supply potential VDD and ground, and a first field effect transistor FET11. The first field effect transistor FET11 has a gate connected to input Vin and a drain connected to output Vout.
The gain controller GC1 is connected between input Vin and output Vout, and includes a second field effect transistor FET12, a capacitor C11, an inductor L12 and a resistor R11. The resistor R11 is connected in parallel between a drain and a source of the second field effect transistor FET12. Besides, the gate of the second field effect transistor FET 12 is connected to control voltage VC1. Detailed description on a variable gain amplifier as shown in FIG. 1 is presented in U.S. Pat. No. 6,285,257.
Reference will now be given to the operation of the conventional variable gain amplifier, in which the amplifier circuit AMP1 of the variable gain amplifier shown in FIG. 1 will not be described since it is well known in the art.
In the gain controller GC1, the resistor R11 maintains uniform voltage level between the drain and the source of the second field effect transistor FET12, and the capacitor C11 isolates voltage at the gate of the first field effect transistor FET11 from power supply potential VDD. Furthermore, by regulating voltage applied to the gate by control voltage VC1, the second field effect transistor FET12 functions as a variable resistor together with the resistor R11. The inductor L12, by enabling the gain controller GC1 to have positive impedance phase, functions to widen the phase range of a feedback signal.
FIG. 2 is an equivalent circuit diagram of the gain controller of the variable gain amplifier shown in FIG. 1, in which the second field effect transistor FET12 and the resistor R11 of the gain controller GC1 are connected in series.
As shown in FIG. 2, the equivalent circuit of the gain controller GC1 is equivalently expressed with the serial connection of a variable resistor R21, a capacitor C21 and an inductor L21.
The conventional variable gain amplifier as shown in FIG. 1 can adjust the resistance of the variable resistor R12 by a control voltage VC1 in order to control amplifier's gain. That is, the gain amplifier boosts up control voltage VC1 to have high gain in response to a small amount of input signal, but reduces control voltage VC1 to have low gain in response to a large amount of input signal.
However, the conventional variable gain amplifier has a field effect transistor as an active device existing on a feedback path for the purpose of gain control. Since the active device has its own resistance, there is a problem in that the FET transistor on the feedback path worsens noise characteristics.
Furthermore, where the conventional variable gain amplifier is applied to a wideband system using two different frequency channels, third order intermodulation frequency originating from intermodulation distortion of the different frequencies takes place in proximity of the channel. Then, such third order intermodulation distortion creates a problem of bad linearity, and the prior art does not teach any means for attenuating third order intermodulation distortion.